Not Applicable.
Not Applicable.
This invention relates to doped-fiber optical amplifiers, and more specifically to doped-fiber optical amplifiers with wavelength-selective optical filters, such as wavelength-division multiplexing (xe2x80x9cWDMxe2x80x9d) filters and gain flattening filters (xe2x80x9cGFF""sxe2x80x9d).
The use of optical networks for data, voice, image and other sorts of transmission is rapidly growing. Optical fiber networks use optical fibers as transmission lines for carrying light signals. The light signals propagate down the fiber from one location to another, analogous to electrical signals traveling down a wire or cable from one location to another. Optical fibers are used in a variety of applications, such as local loops and xe2x80x9clong haulxe2x80x9d transmission lines. Long haul transmission lines might carry signals between cities or across oceans, for example. In other systems, the optical signal might propagate through space, rather than a fiber.
Optical fibers can carry a single channel, or many channels can be multiplexed onto a single fiber. Multiplexing is desirable because it allows more information to be carried on a single fiber. In WDM, a number of channels are carried on a single optical fiber. A channel is typically defined as a frequency (wavelength) of light that can be modulated to carry information. Networks are typically designed to allocate a portion of the spectrum about a center frequency for signal transmission. For example, a channel might be allocated xc2x112.5 GHz on either side of the channel center frequency in a particular system, thus providing the channel with a xe2x80x9cwidthxe2x80x9d of 25 GHz., even though the channels are spaced 100 GHz apart. Other systems may require or allow a narrower or wider channel widths or spacings.
In many optical transmission systems, provisions are made to amplify the signal (channels) at regular intervals. For example, amplification might be provided every 100 km along a long haul optical fiber path to account for signal loss (attenuation) as the optical signal propagates along the optical fiber. Many approaches have been developed to amplify the optical signals in an optical transmission system.
One early approach periodically converted the optical signal to an electronic signal, amplified the electronic signal, and re-generated the optical signal. This approach was cumbersome and required a relatively large number of components, making it prone to failure. It also typically required compromises in the amount of information (bandwidth) that could be amplified through a given path because the upper frequency limit of an electronic amplifier is generally much less than the available frequency bandwidth carried on an optical fiber.
Further approaches utilized light amplifiers to directly amplify the optical signal. There are many types of light amplifiers adaptable for use with optical communication networks; however, a common type is the doped optical fiber amplifier. In a doped optical fiber amplifier, an element(s), such as erbium (Er) is added to the composition of the glass that the fiber is made from. The dopant generally provides energy levels, or states, that can be occupied by photons in the glass fiber. The doped fiber is xe2x80x9cpumpedxe2x80x9d with an external light source, such as a pump laser diode, and this pump light is used to amplify the optical signal.
Erbium-doped fiber amplifiers (xe2x80x9cEDFA""sxe2x80x9d) provide a gain bandwidth suitable for simultaneously amplifying a number of optical channels, and one approach to amplifying the light signal is to amplify all the channels at once in a single broad-band EDFA. This is desirable because it requires only a single amplifier and associated pump circuitry, but has disadvantages, too. First, the gain provided by an EDFA is not uniform for all frequencies (channels). This can result in unequal amplification of the channels, or even an optical signal(s) within a channel. Generally, the greater the bandwidth (i.e. number of channels) amplified by the amplifier, the greater the difference in gain. In a transmission system where amplification is repeated several times between the source and eventual receiver, the difference in signal level between channels could build to unacceptable levels, since the gain difference generally repeats at each amplifier stage.
One way to compensate for the uneven gain is with a GFF. A GFF is generally an optical filter with a transmission characteristic inversely complimentary to the gain response of the amplifier. In other words, the transmission loss through the filter is greatest at the wavelength that is most strongly amplified and is least (preferably essentially zero) at the wavelength having the least amplification.
Second, the gain provided to any one channel is typically somewhat dependant on the total signal the EDFA is amplifying. In other words, the total power needed by the amplifier increases with increasing channel count. For example, at a given pump power, if an EDFA initially amplifies four channels, and the signal traffic on the transmission fiber is expanded to eight channels, the gain of the four original channels will generally be higher than the gain of those channels when the amplifier carries the additional signal traffic. While the light amplifier can sometimes be pumped for higher gain to accommodate the increased number of channels, higher pumping often injects more noise, such as amplified spontaneous emissions (xe2x80x9cASExe2x80x9d), as well as requiring more power from the pump source, often accelerating failure.
One approach to avoid some of the problems associated with broadband amplification of the entire transmission spectrum has been to de-multiplex the transmission spectrum into a number of segments, and to route each segment through an amplifier path. After the segment of the transmission spectrum has been amplified, the optical signals can be sent to a multiplexer and coupled onto the output transmission fiber, or routed to an independent destination, such as a local metro loop.
WDM amplifier systems allow the use of amplifiers optimized for performance over a portion of the band, each amplifier operating more or less independently from the others. However, providing several amplifiers to cover the transmission spectrum increases the component count, and typically increases the volume required for the amplifiers and associated circuitry. Also, WDM filters are relatively expensive, and conventional designs typically employ two filters, one on the output and one on the input. These filters should be closely matched in frequency response to avoid unintended bandwidth narrowing arising from slightly offset center frequencies. The filters should also be matched for thermal drift, or temperature compensated, so that bandwidth narrowing does not arise from changes in the ambient temperature.
Accordingly, an optical amplifier providing matched WDM filtering is desirable. It is further desirable that such an amplifier provide matched thermal drift characteristics to avoid thermally induced bandwidth narrowing. It is further desirable that the amplifier provide integral gain compensation
A four-fiber ferrule assembly is used in an optical amplifier. Doped amplifier fiber sections can be assembled directly into the ferrule(s), or spliced to pigtails. In one application, a four-fiber ferrule contains a pump input fiber for forward pumping one amplifier stage with a pump laser and a pump coupling fiber for forward pumping a second amplifier stage. Alternatively, the pump coupling fiber can be cleaved to provide a second forward pumping input from a second pump laser. The direction of propagation through each amplifier stage is the same, allowing an first optical isolator to be used on the input of both stages and a second optical isolator to be used on the output of both stages.
A single gain-flattening filter is placed in the optical path between the amplifier stages. The output of the second stage is also transmitted through the gain-flattening filter. Thus, a single gain-flattening filter can compensate gain variations in both stages. In a further embodiment, the optical path between amplifier stages includes center stage access for additional optical signal processing, such as dispersion compensation.
In another embodiment, a narrow bandpass filter is placed between the input to the first amplifier stage and a multi-channel optical fiber. The multi-channel optical fiber can also be held in a four-fiber ferrule. The narrow bandpass filter transmits a selected channel(s) to the amplifier input and reflects non-selected or xe2x80x9cexpressxe2x80x9d channels to an output fiber. The amplified selected channel is transmitted back through the narrow bandpass filter and can be coupled onto the output fiber to be re-combined with the express channels, or onto a second output fiber held in the four-fiber ferrule, depending on amplifier configuration.
A second input fiber can carry additional channels that are reflected off the narrow bandpass filter to the second output fiber to be combined with the amplified selected channel. In a further embodiment, an add channel fiber can transmit an add channel (generally occupying the same wavelength as the selected channel) through the narrow passband filter to be coupled with the express channels on the first output fiber.